1. Field of the Invention
The present invention relates to a mobile communication system, a core network node selection method, and a base station and a mobile station used therefor, and more particularly, to a method for selecting a core network node at the time of a handover associated with the movement of a mobile station.
2. Description of Related Art
A mobile communication system based on Long Term Evolution (LTE) of 3GPP aims at such architecture as shown in FIG. 8. This architecture is intended to build a system that can realize data transfer of higher throughput, for example, than existing systems by reducing transmission delay in a control plane and a user plane.
In this architecture, when cells of base stations (eNodes B21 to 23) are small, e.g., in a metropolitan area, it is possible that movement of a mobile station (User Equipment or UE) between base stations significantly increases handover signals transmitted from a base station to a core network node (CN Node 11 or 12, hereinafter abbreviated as CN node). Consequently, excess signal load would be placed on a Core Network (CN). To reduce load of handover signals on the CN node, direct exchange of handover signals between a source base station (Source eNode B) and a target base station (Target eNode B) is under consideration.
Also, as shown in the sequence diagram of FIG. 9 that shows operations during a handover, when a mobile station moves from a cell covered by a source base station to a cell covered by a target base station and the source base station carries out a handover (S1), if a handover is successful by using a handover signal (“Handover Request” message) S2 which the target base station receives from the source base station, that is, if the target base station establishes a communication connection with the mobile station (S3 to S7), the handover procedure can be completed just by the target base station transmitting one signal S8 called “Handover Complete” to a CN node.
The signal name “Handover Complete” is just an example: the signal is also called “Path Switch”, “Binding Update” etc., all of which are supposed to mean the same signal.
Upon receipt of the “Handover Complete” signal, the CN node switches the path for the mobile station from the source base station to the target base station. In this way, the loads of signals and processing on CN nodes due to a handover can be reduced.
Describing this further in detail with reference to FIG. 9, S9 denotes a process for the CN node 11 having received the “Handover Complete” signal S8 to switch the path for the mobile station from the source base station to the target base station. After this process, the CN node 11 transmits “Handover Complete Ack (Acknowledge)” signal S10 to the target base station. Upon receiving the signal S10, the target base station transmits “Release Resource” signal S11 to the source base station, and in response to the signal S11, the source base station releases a radio resource allocated for the UE. The release of resource is notified to the CN node 11 with “Release Resource Indication” signal S12 to complete the handover process. In this manner, loads of signals and processing on CN nodes due to a handover can be reduced.
In a hierarchical structure of CN nodes and base stations, to prevent service unavailability in a wide area covered by one CN node in a core network due to its system failure, for example, such a configuration has been proposed in which CN nodes 11, and eNode Bs 21 to 23, which are base stations, are connected with one another in a mesh configuration so that even if one of the CN nodes fails, the other CN node replaces it to continue to provide services, as shown in FIG. 8. This mesh configuration is called “S1-Flex”, which is known from 3GPP TR 23.236 V6. 3.0 (2006-03) (Non-Patent Document 1). The “S1” denotes S1 interface which is an interface between CN nodes and base stations (eNode B), and “S1-Flex” used to be called “Iu-Flex” in mobile communication systems prior to the LTE system.
In the LTE-based mobile communication system having the S1-Flex configuration, once a mobile station is connected to a CN node, it is basically desirable not to change the CN node as long as the communication continues. This is because it is possible to reduce interruption of communication due to a handover by minimizing handovers between CN nodes that are performed along with the movement of the mobile station, and also because maintaining a communication path for a long time period is desirable for a non-real-time service communication such as data communication.
Existing systems have the soft handover function, which can prevent discontinuity or interruption of audio even when the mobile station moves between base stations. However, the new LTE system mentioned above is designed to build architecture without soft handover because the system is complicated by maintaining the soft handover function.
The new LTE system also aims at the effect of reducing signal load on CN nodes by reducing handovers between CN nodes so that fewer signals are exchanged between CN nodes when a mobile station moves.
However, in the S1-Flex configuration shown in FIG. 8, it is sometimes impractical not to change the CN node with which a mobile station should communicate when the mobile station moves. This is because, by way of example, if a mobile station as a user terminal starts communication in Tokyo and continues to be connected to a CN node in Tokyo even after its user travels to Osaka by Shinkansen, the communication path gets long and transmission delay will significantly increase. In such a case, it is therefore required to change the CN node as the mobile station moves. Consequently, the function to change or not to change a CN node as appropriate when a mobile station moves is required for the S1-Flex configuration.
However, at present, there is no means or way for a target base station to select a CN node at the time of a handover in the LTE system. In the existing system, a CN node receives a handover request signal from a source RNC (radio network controller) and the CN node itself can judge whether to change the CN node.